1. Field of the Invention
Example embodiments generally relate to an image reader, an auto document feeder, and an image forming apparatus, and more particularly, to an image reader for reading an image on a sheet, an auto document feeder (ADF) including the image reader, and an image forming apparatus including the auto document feeder, for example.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium (e.g., a sheet) according to image data generated by reading an image on an original document. Thus, for example, a mobile image reader mounted on a moving mount scans an image on an original document. Alternatively, a stationary image reader scans an image on an original document conveyed at a constant speed.
Such image readers typically also scan a reference board to generate shading data for correction of the image data. In order to generate proper shading data, the reference board must be free of dirt and debris. When the reference board is stained with toner or the like, or some other foreign substance adheres to any part of the reference board, the image reader cannot generate shading data properly.
To address this problem, the mobile image reader on the moving mount can move until the image reader is able to find and scan a clean part of the reference board to generate uncontaminated shading data. Obviously, however, this remedy is not available to stationary image readers, which always generate shading data from the same location. The reference board may be rotatably connected to an arm provided with a motor-driven rotating member to move the reference board to an image sensor only when the image sensor scans the reference board. The reference board separates from the image sensor when the image sensor reads an image on an original document to keep the reference board free from foreign substances originating from the original document. A drawback of this arrangement, however, is that the image sensor has only a shallow depth of focus, and therefore a very small gap must be maintained between the image sensor and the original document. Great precision is needed to rotate the reference board into this small gap. Such precision comes with increased manufacturing costs.
Alternatively, the image reader may be configured to scan a guide roller that is the equivalent of the reference board, divide data obtained by scanning the outer surface of the guide roller for one rotation of the guide roller into a plurality of blocks, and detect a peak value in each block, with the average of the readings of the block having the highest peak value used as shading data. However, when the entire outer surface of the guide roller is stained, the image reader cannot generate proper shading data.